Noise limiting circuit



Nov. 12, 1940. R, G, APP 2,221,700

NOISE LIMITING CIRCUIT Filed Aug. 2, 1939 lm 2 I /6' Patented Nam-12, 1940 PATENT OFFICE NOISE LILHTING CIRCUIT Richard G. Clam. Rutledge, Pa., assig'nor, by mesne assignments, to Pennsylvania Patents, Inc., Carson City, Nev., a corporation of Nevada Application August 2, 1939, Serial No. 288,050 In Great Britain August 26, 1938 7 Claims. (Cl. 250-20) This invention relates to noise limiting circuits for use in electric transmission systems and has for its general object the provision of improved means for limiting the energy and amplitude of interfering noise pulses or signals in carrier wave communication systems and the like.

The invention employs, in a signal transfer path, a non-linear impedance means arranged to operate as a voltage limiter of very short time 10 constant, followed by detection and signal transfer means of relatively long time constant. The non-linear means may be adjusted to limit the amplitude of noise pulses to a value no greater than the maximum amplitude of a desired signal. The succeeding transfer means of long time constant may be designed to just permit the transfer of the important harmonic components in the desired signal, while producing considerable attenuation of sharp'noise pulses of short duration. Such a discriminating means is particularly desirable where the desired signals are relatively weak, and include no important high frequency components, and where/such signals are accompanied by sharp, high-amplitude noise pulses such as those occasioned by atmospherics, autemotive ignition systems, domestic and industrial electrical equipment and the like.

Accordingly, by the present invention, there is provided in an electrical wave transmission system including a source of desired modulated carrier signals accompanied by noise signals, a diode space discharge means for demodulating said carrier signals, said diode means including a resistance-capacity load of relatively large time 35 constant, a control space discharge amplifier means, signal utilization means, a. first coupling means including a resistance-capacity element of large time constant for conductively coupling the input of said amplifier to the output circuit 40 of said diode, a second coupling means including another resistance-capacity element of large time constant for conductively coupling the output of said amplifier and said utilization means, a second diode space discharge device shunted 45 across the signal path, and means for obtaining a biasing voltage for application to the elements of said second diode whereby said second diode acts to limit the amplitude of all noise signals 50 to a predetermined amplitude, the time constant of said resistance-capacity diode load, and said first and second resistance-capacity coupling means, being of such values that said noise signals are greatly attenuated while said desired 55 signals are transferred with no substantial loss.

' attenuated:

A further object of the invention is to provide a circuit for efiiciently demodulating and transferring relatively slow control signals, while 15 greatly attenuating, and inefliciently demodulating and transferring undesired noise signals.

The invention may be understood by reference to the accompanying drawing, the single figure of which is a schematic illustration of an em- 0 bodiment of theinvention;

In the figure, E represents a source of modulated carrier signals which may be accompanied by noise signals. In the signal path between the source E and a suitable utilization means I! there are disposed a plurality of space discharge elements, which if desired may be enclosed in a single envelope, and which may comprise at least two non-linear impedance means and a control space discharge device such as the diode elements 4 and 5 and a pentode element or the like. Disregarding for the moment the presence of the limiter diode element 4, the signal path is seen to include a demodulating diode element 5 whose load includes the resistor 1. This diode may be coupled to the signal source by means of a timed transformer 3 and the condenser 6. For D. C. and low frequencies, the condenser 6 is effectively-in shunt with the resistor i, and hence the diode load is in fact formed by the shunt combination of the elements 6-1. The demodulated signal may be taken from the high, or anode, side of the resistor I, and in this particular embodiment is impressed on the pentode control grid by means of the resistancecapacitance network l2l3; this network serves not only as an r-f filter, but also as a part of the means provided by the invention for discriminating against noise pulses as hereinafter so explained.

The output load of the pentode may be a resistor l4. Coupling to the utilization device I! may be eflected by means of a resistancecapacitance circuit l5-I6. Preferably, the time constant R-C of this network should be so large that relatively short, sharp, noise signals are greatly attenuated, but not so much as to efiect important changes in the amplitude and/or Wave-shape of the desired demodulated signal. For similar reasons the time constant of the coupling network l2-|3 may be of the same order, or it may be somewhat smaller.

The time constant of the efiective diode load 6'! should likewise be relatively large; in general of such a magnitude as to provide efilcient detection of the desired signal or portion of the signal, but very inefficient detection of noise pulses of short duration. Thus, it will be appreciated that in the case of those noise-pulse frequencies for which the reactance of the condenser 6 is small compared to the resistance 1, the signals will be largely shunted to ground by way of the condenser 6 and the secondary of the transformer 3. Accordingly, the diode load 6-! not only forms a part of the demodulator circuit, but also acts as an efiective discriminator 'against certain undesired signals.

In addition to the protective means hereinbefore described, there is provided the diode limiter element 4 which may be suitably connected in shunt with some portion of the signal transfer path, such as the secondary of the transformer 3. The diode anode may be supplied with a negative bias or delay voltage by means of a battery or by means of the cathode resistor l0, which may be shunted by a by-pass condenser 8. If the latter form of bias is selected, care should be taken to assure that the parallel circuit shall have a very short time constant and a low impedance in order that the delay voltage may be steady, and not subject to erratic excursions as a result of heightened noise signal presence. Preferably, the delay voltage is adjusted to the order of the amplitude of the desired signal, thus assuring that the limiter may act to limit the amplitude of noise pulses to that of the desired signal.

The system herein disclosed has proved particularly efilcient in discriminating against unwanted noise pulses in the reception of the relatively slow control pulse signals employed in certain types of remote control apparatus. Such a remote control system has been described in copending application of Milton L. Thompson, Serial No. 220,356, filed July 20, 1938. When the circuit of the present invention was used in place of the detector-amplifier circuit of the remote control signal amplifier described in the abovementioned copending application a very considerable improvement in the signal-noise ratio was obtained. Obviously the invention is not limited to such systems, but may be applied to carrier wave communication systems in general.

It is also obvious that the invention itself is capable of various modifications and is by no means limited to the specific form which has been illustrated and described for the purposes of disclosure. Any such modifications are deemed to be within the scope of the invention.

I claim:

1. A noise limiting circuit for reducing the amplitude of noise pulses in carrier wave transmission systems, comprising a source of carrier signals, means for demodulating said signals, said demodulating means including a resistance-capacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulating means to the input circuit of said amplifier, means coupling the output of said amplifier and said utilization means, a device having a nonlinear current-voltage characteristic shunted across the signal path, means for applying a biasing voltage to the elements of said non-linear device whereby said device tends to limit the amplitude of noise signals to a predetermined amplitude, the time constant of said demodulator load, and of said resistance-capacity coupling means, being of such high values that said noise signals are greatly attenuated, while the desired signals are transferred with no substantial loss.

2. A circuit according to claim 1 wherein the control space discharge amplifier is a pentode.

3. A noise limiting circuit for reducing the amplitude of noise pulses in carrier wave transmission systems, comprising a source of carrier signals, means for demodulating said signals, said demodulating means comprising a diode and a resistance-capacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulating means to the input circuit of said amplifier, means coupling the output of said amplifier and said utilization means, a device having a non-linear current-voltage characteristic shunted across the signal path, means for applying a biasing voltage to the elements of said non-linear device whereby said device tends to limit the amplitude of noise signals to a predetermined amplitude, the time constant of said demodulator load, and of said resistance-capacity coupling means, being of such high values that said noise signals are greatly attenuated, while the desired signals are transferred with no substantial loss.

4. A noise limiting circuit for reducing the amplitude of noise pulsess in carrier wave transmission systems, comprising a source of carrier signals, means for demodulating said signals, said demodulating means including a resistancecapacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulating means to the input circuit of said amplifier, means coupling the output of said amplifier and said utilization means, a diode shunted across the signal path, means for applying a biasing voltage to the elements of said last-mentioned diode whereby the same tends to limit the amplitude of noise signals to a predetermined amplitude, the time constant of said demodulator load, and of said resistance-capacity coupling means, being of such high values that said noise signals are greatly attenuated, while the desired signals are transferred with no substantial loss.

5. A noise limiting circuit for reducing the amplitude of noise pulses in carrier wave transmission systems, comprising a source of carrier signals, means for demoduating said signals, said demodulating means including a resistance-capacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulaitng means to the input circuit of said amplifier, coupling means including a resistance-capacity element of relatively large time constant for coupling the output of said amplifier and said utilization means, a device having a non-linear current-voltage characteristic shunted across the signal path, means for applying a biasing voltage to the elements of said non-linear device whereby said device tends to limit the amplitude of noise signals to a predetermined amplitude, the time constant of said demodulator load, and of said resistance-capacity coupling means, being of such high values that said noise signals are greatly attenuated, while the desired signals are transferred with no substantial loss.

6. A noise limiting circuit for reducing the amplitude of noise pulses in carrier wave transmission systems, comprising a source of carrier signals, means for demodulating said signals, said demodulating means including a resistancecapacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, conductive coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulating means to the input circuit of said amplifier, conductive coupling means including a resistance-capacity element or relatively large time constant for coupling the output of said amplifier and said utilization means, a device having anon-linear current voltage characteristics shunted across the signal path, means for applying a biasing voltage to the elements of said non-linear device whereby said device tends to limit the amplitude of noise signals to a predetermined amplitude, the time constant of said demodulator load, and of said resistance-capacity coupling means, being of such high values that said noise signals are greatly attenuated, while the desired signals are transferred with no substantial loss.

'7. A noise limiting circuit for reducing the amplitude of noise pulses in carrier wave transmission systems, comprising a source of carrier signals, means for demodulating said signals, said demodulating means including a resistancecapacity load of relatively large time constant, a control space discharge amplifier, signal utilization means, coupling means including a resistance-capacity element of large time constant for coupling the output circuit of said demodulating means to .the input circuit'of said amplifier, means coupling the output of said amplifier and said utilization means. a device having a non-linear current-voltage characteristic shunted across the signal path, means for applying a biasing voltage to the elements of said non-linear device whereby said device tends to limit the amplitude of noise signals to a predetermined amplitude, said biasing means including a resistance-capacity elementhaving a 25 said noise signals are greatly attenuated, while the desired signals are transferred with no sub stantial loss.

RICHARD G. CLAPP. 

